Nuclear power reactors are well known and are discussed, for example, by M. M. El-Wakil in "Nuclear-Power Engineering" McGraw-Hill Book Company Inc., 1976.
In a known type of nuclear power reactor, for example, as used in the Dresden I reactor of the Dresden Nuclear Power Station near Chicago, Ill., the reactor core is of the heterogenous type. In such reactors the nuclear fuel comprises elongated rods formed of sealed cladding tubes of suitable material, such as zirconium alloy, containing uranium oxide and/or plutonium oxide as the nuclear fuel, for example, as shown in U.S. Pat. No. 3,365,371. A number of such fuel rods are grouped together and contained in an open-ended tubular flow channel to form a separately removable fuel assembly or bundle as shown, for example, in U.S. Pat. No. 3,431,170. A sufficient number of fuel assemblies are arranged in a matrix, approximating a right circular cylinder, to form the nuclear reactor core capable of self-sustained fission reaction. The core is submerged in a fluid, such as light water, which serves both as a coolant and as a neutron moderator.
A typical fuel assembly is formed by an array of spaced fuel rods supported between upper and lower tie plates, the rods being several feet in length, on the order of one-half inch in diameter and spaced from one another by a fraction of an inch. To provide proper coolant flow past the fuel rods it is important to maintain the rods in spaced position and restrain them from bowing and vibrating during reactor operation. A plurality of fuel rod spacers spaced along the length of the fuel assembly are provided for this purpose. A variety of such fuel rod spacers have been proposed and used.
Design considerations of such fuel rod spacers include the following: retention of rod-to-rod spacing; retention of fuel assembly shape; allowance for fuel rod thermal expansion; restriction of fuel rod vibration; ease of fuel bundle assembly; minimization of contact areas between the spacer and fuel rods; maintenance of structural integrity of the spacer under normal and abnormal (such as seismic) loads; minimization of reactor coolant flow distortion and restriction; maximization of thermal limits minimization of parasitic neutron absorption; minimization of manufacturing costs including adaptation to automated production. Thus the need to provide such fuel rod spacers creates several significant problems three of which are parasitic neutron absorption, thermal limits and coolant flow restriction or pressure drop.
Any material, in additon to the nuclear fuel, that must be used in the construction of the reactor core unproductively absorbs neutrons and thus reduces reactivity with the result that an additional compensating amount of fuel must be provided. The amount of such parasitic neutron absorption is a function of the amount of the non-fuel material, of its neutron absorption characteristics, that is, its neutron absorption cross section, and of the neutron flux density to which it is exposed.
To remove the heat from the nuclear fuel, pressurized coolant is forced through the fuel assemblies of the reactor core. The fuel rod spacers in the assemblies act as coolant flow restrictors and cause an undesirable though inevitable coolant flow pressure drop. To maintain proper cooling of the fuel rods along their length and to minimize the required coolant pumping power it is desirable that spacer coolant flow restriction be minimized. The flow restriction of a spacer is a strong function of its projected or "shadow" area. Therefore, the flow restriction of a spacer can be minimized by minimizing the projected area of the structure of the spacer. Tests have shown that spacers employing minimized projected area also have the highest thermal limits.
As a practical matter the desire to minimize both parasitic neutron absorption and coolant flow restriction presents a conflict in fuel rod spacer design.
To minimize coolant flow restriction, spacer members must be thin and of minimal cross section area. However, such thin members must be formed of high strength material having suitable resiliency characteristics. It is found that suitable such materials have relatively high neutron absorption characteristics.
On the other hand, materials of desirably low neutron absorption characteristics are found to be of relatively low strength, difficult to form and lacking the resiliency desired for the spring member portions of the spacer.
The foregoing design conflict has resulted in two distinguishably different approaches to spacer design. A first design approach is a "composite" spacer formed of relatively large structural members from a material having a low neutron absorption cross section and fitted with separately formed spring members of suitably resilient material whereby the amount of high neutron absorption cross section material is minimized. This first type of spacer thus provides minimal neutron absorption but relatively high coolant flow resistance.
A second design approach is a spacer with a highly skeletonized structure using a minimum of a high strength material of suitable resiliency but having a higher neutron absorption cross section. This second type of spacer thus provides minimal coolant flow resistance but at the expense of higher neutron absorption.
The composite type of spacer is exemplified, for example, by the disclosure of U.S. Pat. No. 3,654,077. The skeletonized type of spacer is exemplified, for example, by the disclosures of British Pat. No. 1,480,649 and U.S. Pat. No. 4,190,494.
An object of the invention is to improve nuclear reactor performance by a spacer arrangement which provides an advantageous compromise between reducing parasitic neutron absorption and minimizing coolant flow restriction thereby maximizing both thermal limits and pressure drop performance.
Another object is a spacer arrangement which takes advantage of the different neutron flux density regions of a boiling water nuclear reactor.